It is sometimes necessary or desirable to measure the presence and/or concentration of blood constituents, such as blood gases, hydrogen ions other electrolytes, glucose, red blood cells and the like. This can be accomplished continuously and in real time in a patient undergoing surgery or intensive care by utilizing a catheter and a probe. The probe includes one or more sensors which are responsive to the constituent or compositional parameter of interest to provide a signal related to such constituent. One such system is shown in Maxwell U.S. Pat. No. 4,830,013.
According to the Maxwell patent, the accuracy of the blood constituent measurement system can be improved by keeping the sensor from contacting the wall of the vessel into which the catheter and probe are inserted. In one form illustrated in the Maxwell patent, this is accomplished by positioning the sensor within the lumen of the catheter and adjacent the distal opening of the catheter.
In another blood gas measurement system shown in Nestor et al U.S. Pat. No. 4,900,933, a dual-lumen catheter is provided with one lumen being used for fluid infusion and the other lumen being used for the probe. In the Nestor et al construction, the probe and the sensor of the probe are located outside the lumen distally of the distal end of the lumen. Accordingly, this sensor would appear to be subject to providing inaccurate readings as a result of contact with the wall of the vessel into which it is inserted and also appear to be subject to thrombus deposition of, for example, protein and platelets. In addition, the sensor would also appear to be relatively difficult to clean.